There is known a differential transmission system adapted to transmit a differential signal pair, comprising signals having opposite phases, in two signal lines forming a pair. Since the differential transmission system has a feature that the data transfer rate can be made high, it has recently been put to practical use in various fields.
For example, in the case of using the differential transmission system for data transfer between a device and a liquid crystal display, the device and the liquid crystal display are each provided with a display port connector which is designed according to the display port standard. As this display port standard, VESA DisplayPort Standard Version 1.0 or its Version 1.1a is known.
This display port connector is a kind of differential signal connector and has a first connection side for connection to a connection partner and a second connection side for connection to a board of the device or the liquid crystal display. The configuration of the first connection side is strictly defined by the display port standard in terms of the relationship with the connection partner while the configuration of the second connection side is relatively free. This type of differential signal connector is disclosed in Patent Document 1 (JP-A-2008-41656).
FIGS. 1A and 1B show a contact assembly 1 incorporated in a conventional differential signal connector which is different from the one disclosed in Patent Document 1 but is similar in configuration thereto. The contact assembly 1 comprises a plurality of pairs of signal contacts 2, a plurality of ground contacts 3, and an insulating housing 4 holding the signal contacts 2 and the ground contacts 3. On the first connection side for connection to a connection partner, the ground contacts 3 are arranged on both sides of each pair of signal contacts 2 so that a fixed-pitch contact array is formed. On the other hand, on the second connection side for connection to a board, the signal contacts 2 and the ground contacts 3 are bent in a direction crossing the contact array so that the signal contacts 2 and the ground contacts 3 are arranged zigzag in two rows.
FIG. 2 shows a board 5 for mounting thereon the differential signal connector including the contact assembly 1 of FIGS. 1A and 1B. The board 5 is formed with a plurality of through holes 6. The through holes 6 are arranged zigzag in two rows so as to correspond to the arrangement of the signal contacts 2 and the ground contacts 3 on the second connection side.
When the differential signal connector is mounted on the board 5, the signal contacts 2 and the ground contacts 3 are respectively inserted into the through holes 6. Lands 7 each in the form of a doughnut-shaped conductor pattern are respectively formed around openings of the through holes 6. Further, wiring patterns 8 are drawn out in parallel along the board 5 from only those lands 7 which are formed corresponding to the through holes 6 adapted to be inserted with the signal contacts 2. Therefore, each signal contact 2 is connected to the wiring pattern 8 through the through hole 6 and the land 7.
In the above-mentioned differential signal connector, arranging the signal contacts and the ground contacts zigzag in two rows on the second connection side, itself, easily makes it possible to reduce the size of the connector. However, if the connector is actually reduced in size this way, there arise the following problems due to the occurrence of a difference in length between the differential signal contacts forming a pair.
As shown in FIG. 3, a plurality of pairs of signal contacts and a plurality of ground contacts can be collectively manufactured by punching a single conductor plate and then carrying out bending. In order to facilitate this manufacturing process, it is common sense of those skilled in the art that forward ends of the contacts are arranged at regular intervals in a bent state and that the number of times of contact bending is set to two. However, in order to arrange the forward ends of the contacts at regular intervals in the bent state, there occurs a difference in length between the differential signal contacts forming a pair as is well seen from a developed state of the contacts shown in FIG. 3. This difference in length causes a propagation time difference (skew) between a differential signal pair in a differential signal connector.
Further, due to this difference in length, there is a case where, on the second connection side, i.e. on a board, the differential signal contacts forming a pair are separated in two rows, i.e. not arranged in the same row. This also applies to the ground contacts arranged on both sides of such a pair of differential signal contacts. In this case, there occurs a difference in length between a pair of wiring patterns connected to such a pair of differential signal contacts, as is also seen from FIG. 2 where there are shown the wiring patterns with different lengths which are drawn out from the lands formed in different rows. This difference in length between the pair of wiring patterns also causes a skew between a differential signal pair.